Enhance serialization documentation with details on server function payloads handling

Author: LadyBluenotes

src/routes/solid-start/v2/(1)advanced/(4)serialization.mdx

@@ -40,6 +40,20 @@ export default defineConfig({
 
 If your app enforces a Content Security Policy, keep `json`.
 
+## Server function payloads
+
+SolidStart applies extra handling for certain payload types so file uploads and binary data can flow without being serialized by Seroval. This applies to both server function arguments and return values. SolidStart bypasses Seroval for:
+
+- `FormData`
+- `URLSearchParams`
+- `Uint8Array`
+- `ArrayBuffer`
+- `Blob`
+- `File`
+- `string`
+
+Because these values are transferred directly, this can yield smaller payloads for these cases.
+
 ## Related
 
 - [Data fetching](/v2/solid-start/building-your-application/data-fetching)